why having small body is treated like a bad thing

im 20f always keep getting weird questions or advices since i was a kid
as why are you so slim ? you need to go to the gym and grow your butt . why are your melons small?
i always been a slim kid and now i kinda have a fat ahh but it's only cause i dont go out much but my melons are B2 or36/38 size im not sure
when i went to sleep over with my cousin she mentions why dont i wear push up bras or why i dont wear bras at home even though they arent big
idk i prefer comfy stuff and my mom didnt really teach me what i should do and idk what to search i know the basics of personal care
should i really wear push up bras outside? i have one but i didnt feel any diffrence and it's not comfy
note : it's not always there topic but i get these comments and i usually feel bad after and idk what to say i just say idc or i prefer comfort
i usually wear dantel bras